The invention generally relates to an automatic gain control apparatus and technique.
The strength of a signal that is received by a radio receiver may significantly vary, depending on such factors as the distance between the source and the receiver, the presence of physical obstructions, the presence of out-of-band blockers, etc. Therefore, a conventional radio receiver includes automatic gain control (AGC) circuitry for purposes of regulating the amplitude of the signal that is processed by the circuitry of the receiver.
A conventional AGC stage may include a differential amplifier that has a differential pair of transistors, which receives a control signal for purposes of controlling the stage's gain. The input signal to the AGC stage may, for example, modulate the tail current of the differential amplifier. Thus, via a gain established by the control signal, the differential amplifier amplifies the input signal to produce the AGC stage's output signal. A conventional AGC stage may include a differential amplifier that has a differential pair of transistors, which receive an input signal at the input, base, terminals. The amplifier stage provides transconductance gain to the input signal outputting a differential current whose amplitude is dependent on the input signal. The output signal may be connected to the common emitter terminal of a second differential pair of transistors, which receive a control signal at the input, base, terminals. The second differential pair will provide a variable output current level and so gain to one of the second differential pair's output, collector, terminals by steering current to or away from said terminal. A conventional AGC stage, such as the above-described AGC stage, may have relatively insufficient noise figure (NF) and/or intermodulation (IM) characteristics, over part or all of the gain control range.
Thus, there exists a continuing need for better ways to control the gain of a signal in a radio receiver.